1. Field of the Invention
The present invention generally relates to a coordinate detecting device, a writing instrument, and a coordinate inputting system. The present invention specifically relates to a coordinate detecting device that is suited for detection of a trajectory of a pen tip on a paper.
2. Description of the Related Art
Conventionally, a following coordinate detecting device for inputting coordinates is known (refer to Japanese Patent Application Laid-open Nos. 2002-509317, 2004-70887, and S54-137921). The coordinate detecting device inputs the coordinates by disposing a pair of ultrasonic receivers on a paper, using the ultrasonic receivers to receive ultrasonic waves transmitted from a writing instrument, and estimating a position of the writing instrument from propagation times of the ultrasonic waves using triangulation.
In the coordinate detecting device described in Japanese Patent Application Laid-open No. 2002-509317, two ultrasonic transmitters are arranged in the pen. The two ultrasonic transmitters simultaneously transmit ultrasonic waves at respectively different transmitting frequencies. The coordinate detecting device includes a detector and a unit for estimating the position of the pen tip. The detector receives the ultrasonic waves transmitted from the two ultrasonic transmitters. The unit calculates positions of the ultrasonic transmitters at a given time by separating two frequencies and estimates the correct pen tip position from the calculated result.
In the coordinate detecting devices described in Japanese Patent Application Laid-open Nos. 2004-70887 and Japanese Patent Application Laid-open No. S54-137921, two ultrasonic sources are arranged in the pen and two ultrasonic receivers are arranged in the coordinate detecting device. The ultrasonic sources output ultrasonic waves and the ultrasonic receivers receive the ultrasonic waves output from the pen. The ultrasonic sources alternately emit the ultrasonic waves. The coordinate detecting device measures propagation times of the ultrasonic waves received by the ultrasonic receiver and estimates the position the pen tip from a difference between the propagation times.
However, the coordinate detecting device described in Japanese Patent Application Laid-open No. 2002-509317 requires a device, such as a filter, in the coordinate detecting device to discriminate the two frequencies. Therefore, not only does the coordinate detecting device become costly, but calculation of a correct reception time becomes difficult because the use of the filter causes changes in phases of the received ultrasonic waves.
In the coordinate detecting devices described in Japanese Patent Application Laid-open Nos. 2004-70887 and S54-137921, transmission times of the ultrasonic waves transmitted from the ultrasonic sources differ so that a time difference occurs between time points at which the coordinate detecting device calculates positions of the ultrasonic sources mounted on the pen. If an inclination of the pen changes or the pen tip moves during the time difference, accurate position of the pen tip can not be calculated.
Specifically, the problem that occurs when the inclination of the pen changes or the pen tip moves during the time difference is explained below with reference to FIG. 20 and FIG. 21.
FIG. 20 is a schematic for explaining estimation of a pen tip position. The pen tip is placed on point P0, the pen (not shown) is tilted and rotated at a constant speed around the point P0 without moving the pen tip. The distance between the pen tip and one ultrasonic source (hereinafter, “speaker A”) and the distance between the speaker A and another ultrasonic source (hereinafter, “speaker B”) are equal.
FIG. 21 is a diagram of pen-tip interpolation times when speaker positions are interpolated. The diagram shows a relationship among a time represented by a solid circle, a time represented by an open circle, and a time represented by an open square. The time represented by the solid circle is when the ultrasonic waves can be received and the speaker positions can be actually identified. The time represented by the open circle is when the positions of the speaker A and the speaker B are acquired by interpolation being performed from a time at which two consecutive positions of the speaker A and the speaker B can be acquired. (However, in actuality, a speaker position corresponding to the time is needed but impossible to acquire. Therefore, the speaker positions are replaced with an inner ratio of the two speaker positions.) The time represented by the open square is when pen tip coordinates can be estimated from the interpolated position of the speaker A and the interpolated position of the speaker B.
In FIG. 21, a horizontal axis is a time axis. The speaker A and the speaker B transmit the ultrasonic waves at a cycle T. The speaker B transmits the ultrasonic waves with a time lag of T/2 cycle from when the speaker A transmits the ultrasonic waves. In FIG. 20, a solid circle on a circumference indicates an acquired speaker position. An open circle on a dotted line indicates an interpolation point of the acquired speaker position used to determine the pen tip position. A cross P0 indicates a point used to estimate a true pen tip position. Small solid circles P0′ and P0″ near the cross indicate estimated pen tip positions.
Specifically, a method for estimating the pen tip position using the interpolation of the speaker positions will be described with reference to FIG. 20. In FIG. 20, P11, P12, and P13 indicate speaker positions when the position of the speaker A is measured at a certain time interval T. If P11 is acquired at time t, P11 is acquired at time t+T, and P13 is acquired at time t+2T.
P21, P22, and P23 indicate the positions of the speaker B when the position of the speaker B is measured with a half-cycle (T/2) time lag from the cycle at which the position of the speaker A is measured. If P21 is acquired at time t+T/2, P22 is acquired at time t+3T/2, and P23 is acquired at time t+5T/2.
First, a pen tip position (P0″) is estimated using P22 and P1(tc2), with P1(tc2) as a center point between P12 and P13. When a line segment connecting P22 and P1(tc2) is extended and P0″ is determined so that a length between P1(tc2) and P0″ is the same as a length between P22 and P1(tc2), the determined P0″ is an estimated pen tip position P0″.
Next, a pen tip position (P0′) is estimated using P2(tc2) and P12, with P2(tc2) as a center point between P21 and P22. When a line segment connecting P2(tc2) and P12 is extended and P0′ is determined so that a length between P12 and P0′ is the same as a length between P2(tc2) and P12, the determined P0′ is an estimated pen tip position P0′.
However, as is clear in FIG. 20, the estimated results of P0′ and P0″ are positioned away from the true P0. The reason is because one point used to estimate the position of the pen tip is a measured speaker position with few errors, while the other point is an interpolated speaker position with significant errors. In this way, in the conventional methods, pen-tip position estimation errors become extremely large.